The measurement strategies that have been used so far to study the relationship between alcohol or drug use and high risk sexual behavior have failed to provide a clear understanding of the strength or meaning of this link. Although many behavioral studies have detected a relationship between alcohol or drug use and high risk sexual behavior, other studies have not. Reviews of these divergent findings have focused on measurement differences between studies, and these differences appear to have influenced the ability of behavioral studies to detect a relationship between high risk sexual behavior and concurrent drug or alcohol use. We propose to field a measurement strategy that will overcome problems encountered so far in measuring sexual behavior under the influence of alcohol or drugs. We have adapted a previous advance in the measurement of alcohol consumption so that it also measures sexual behavior and drug use at the level of the sexual event (e.g. The Timeline Followback Method; Sobell, et al., 1980). An important advantage of this new measurement strategy is that it measures alcohol or drug use within the context of all sexual events for a defined period of time. Thus, this strategy has the advantage of yielding data that measure the contexts surrounding a series of sexual events while also measuring characteristics of the individual, so the explanatory models can be build that take both kinds of information into account in predicting whether high risk sexual behaviors occur within a defined period of time. Finally, we will use the Timeline Followback interview as the basis for a qualitative assessment of the reasons, identified by the respondents themselves, for engaging in safe and unsafe sexual behaviors. We believe that the important measurement and analytical advantages afforded by the Timeline Followback Method will contribute in important ways to resolving the controversy that currently exists in the behavioral literature regarding the contributions of drug/alcohol use to high risk sexual behavior.